The present invention relates to an electrophotographic copier, facsimile apparatus, laser printer or similar image forming apparatus and, more particularly, to a developing device of the type developing a latent image electrostatically formed on a photoconductive element with a developer implemented by a developing liquid.
An image forming apparatus including a developing device of the type using a highly viscous and dense developing liquid is taught in, e.g., Japanese Patent Laid-Open Publication No. 7-209922 or 7-239615. The conventional apparatus of this type includes a prewetting liquid applying device for applying to a photoconductive element a dielectric prewetting liquid which has a parting ability and is chemically inactive, e.g., dimethyl polysiloxane oil. The photoconductive element with the prewetting liquid is charged and then exposed so as to electrostatically form a latent image thereon. The latent image is developed by a developing device using a highly viscous developing liquid made up of a solvent implemented by dimethyl polysiloxane oil or similar insulating liquid and toner particles densely dispersed in the solvent. The developing liquid is applied to a belt, roller or similar developer carrier in the form of a thin layer by a developer applying device.
A problem with the above developing device is that the developer applying device cannot form a thin uniform liquid layer on the developer carrier without needing several application rollers or even several tens of application rollers. Such a number of application rollers result in a complicated construction and obstruct easy maintenance. Should the number of application rollers be reduced for a simple configuration, a number of ribs would occur on the surface of the thin viscous liquid layer formed on the developer carrier and would make it difficult to uniform the thin liquid layer. The nonuniform liquid layer on the developer carrier degrades the effect available with the prewetting liquid, i.e., the obviation of background contamination of the photoconductive element at the time of development, and fails to develop an image uniformly.
It is also a common practice with the developing device using a developing liquid to collect the developing liquid and prewetting liquid remaining on the developer carrier in a reservoir so as to reuse them. However, repeating the collection of the developing liquid and prewetting liquid is undesirable from in the following respect. The prewetting liquid sequentially increases in amount in the developing liquid used to develop a latent image formed on the photoconductive element. As a result, the toner content, viscosity and other factors of the developing liquid vary and cause the conditions for forming the thin layer on the developer carrier and the amount of toner to deposit on the photoconductive element to vary, deteriorating the image quality.